Electrochemical cells, usually classified as fuel cells, are devices used for generating electric current from chemical reactions. Fuel cell technology offers a promising alternative to traditional power sources for a range of technologies, for example, transportation vehicles and portable power supply applications. A fuel cell converts the chemical energy of a fuel (e.g., hydrogen, natural gas, methanol, gasoline, etc.) into electricity through a chemical reaction with oxygen or other oxidizing agent. The chemical reaction typically yields electricity, heat, and water. A basic fuel cell comprises a negatively charged anode, a positively charged cathode, and an ion-conducting material called an electrolyte.
Different fuel cell technologies utilize different electrolyte materials. A Proton Exchange Membrane (PEM) fuel cell, for example, utilizes a polymeric ion-conducting membrane as the electrolyte. In a hydrogen PEM fuel cell, hydrogen atoms are electrochemically split into electrons and protons (hydrogen ions) at the anode. The electrochemical reaction at the anode is: 2H2→4H++4e−.
The electrons produced by the reaction flow through an electric load circuit to the cathode, producing direct-current electricity. The protons produced by the reaction diffuse through the electrolyte membrane to the cathode. An electrolyte can be configured to prevent the passage of negatively charged electrons while allowing the passage of positively charged ions.
Following passage of the protons through the electrolyte, the protons can react at the cathode with electrons that have passed through the electric load circuit. The electrochemical reaction at the cathode produces water and heat, as represented by: O2+4H++4e−→2H2O.
In operation, a single fuel cell can generally generate about 1 volt. To obtain the desired amount of electrical power for a particular application, individual fuel cells are combined to form a fuel cell stack. The fuel cells are stacked together sequentially, each cell including a cathode, an electrolyte membrane, and an anode. Each cathode/membrane/anode assembly constitutes a “membrane electrode assembly” (MEA), which is typically supported on both sides by bipolar plates. Gases (hydrogen and air) are supplied to the electrodes of the MEA through channels or grooves formed in the plates, which are known as flow fields. In addition to providing mechanical support, the bipolar plates (also known as flow field plates or separator plates) physically separate individual cells in a stack while electrically connecting them. The bipolar plates can also act as current collectors, provide access channels for the fuel and the oxidant to the respective electrode surfaces, and provide channels for the removal of water formed during operation of the cell. Typically, bipolar plates are made from metals, for example, stainless steel, titanium, etc., and from non-metallic electrical conductors, for example, graphite.
Additionally, a typical fuel cell stack includes manifolds and inlet ports for directing the fuel and oxidant to the anode and cathode flow fields, respectively. The stack may also include a manifold and inlet port for directing a coolant fluid to interior channels within the stack to absorb heat generated during operation of the individual cells. A fuel cell stack also includes exhaust manifolds and outlet ports for expelling the excess gases and the coolant water.
FIG. 1 is an exploded schematic showing the various components of a PEM fuel cell 10. As shown, bipolar plates 2 flank the MEA, which comprises an anode 7A, a cathode 7C, and an electrolyte membrane 8. Hydrogen atoms supplied to anode 7A are electrochemically split into electrons and protons (hydrogen ions). The electrons flow through an electric circuit (not shown) to cathode 7C and generate electricity in the process, while the protons move through electrolyte membrane 8 to cathode 7C. At the cathode, protons combine with electrons and oxygen (supplied to the cathode) to produce water and heat.
Additionally, PEM fuel cell 10 comprises electrically-conductive gas diffusion layers (GDLs) 5 within the fuel cell on each side of the MEA. GDLs 5 serve as diffusion media enabling the transport of gases and liquids within the cell, provide electrical conduction between bipolar plates 2 and electrolyte membrane 8, aid in the removal of heat and process water from the cell, and in some cases, provide mechanical support to electrolyte membrane 8.
In a typical fuel cell, reactant gases on each side of the electrolyte membrane flow through the flow fields and then diffuse through the porous GDL to reach the electrolyte membrane. Since the flow field and the GDL are positioned contiguously and are coupled by the internal fluid streams, the flow field and the GDL are collectively referred to as “flow structure” hereinafter, unless specified otherwise. It is, however, within the scope of the present disclosure to use traditional channel-type flow fields in combination with three-dimensional porous metallic GDLs, to use three-dimensional porous metallic flow fields in combination with traditional GDLs, or to use three-dimensional porous metallic substrates as both flow fields and GDLs.
The reactant gases on each side of the electrolyte membrane are often present at different pressures, therefore a pressure differential is created across the MEA. The pressure differential creates a force on the MEA that causes the MEA to move away from the high pressure toward the low pressure. A consequence of this movement can be a reduction in contact pressure and separation of the contacting surface of the MEA from the flow structure on the high pressure side. It is believed that reduction in pressure and subsequent separation between the contacting surfaces of the MEA and the high pressure flow structure reduces the electrical conduction and increases the contact resistance between the two reducing the efficiency of the fuel cell. Reduction in contact pressure and separation due to high pressure operation has created a continuing need to improve the design of the flow structures for electrochemical cells to overcome this inefficiency.
The present disclosure is directed toward the design of improved flow structures for use in electrochemical cells. In particular, the present disclosure is directed towards the design of resilient flow structures for use with electrochemical cells. Such devices may be used in electrochemical cells operating under high differential pressures, including, but not limited to fuel cells, electrolysis cells, hydrogen purifiers, hydrogen expanders, and hydrogen compressors.